From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Date: Wed, 12 Sep 2007 23:00:37 +0000 Subject: Re: [PATCH] local_t protection (critical section) Message-Id: <20070912230037.GA1571@Krystal> List-Id: References: <20070827211003.GA10627@Krystal> <20070827213845.GB9748@Krystal> <20070827222715.GA16982@Krystal> <20070904200429.GD8157@Krystal> <20070905130411.GB11880@Krystal> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Christoph Lameter Cc: Peter Zijlstra , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, mingo@redhat.com, linux-ia64@vger.kernel.org * Christoph Lameter (clameter@sgi.com) wrote: > On Wed, 5 Sep 2007, Mathieu Desnoyers wrote: > > > Index: linux-2.6-lttng/include/asm-generic/local.h > > =================================> > --- linux-2.6-lttng.orig/include/asm-generic/local.h 2007-09-04 15:32:02.000000000 -0400 > > +++ linux-2.6-lttng/include/asm-generic/local.h 2007-09-05 08:50:47.000000000 -0400 > > @@ -46,6 +46,15 @@ typedef struct > > #define local_add_unless(l, a, u) atomic_long_add_unless((&(l)->a), (a), (u)) > > #define local_inc_not_zero(l) atomic_long_inc_not_zero(&(l)->a) > > > > +#define local_enter_save(flags) local_irq_save(flags) > > +#define local_exit_restore(flags) local_irq_restore(flags) > > +#define local_enter() local_irq_disable() > > +#define local_exit() local_irq_enable() > > +#define local_nest_irq_save(flags) (flags) > > +#define local_nest_irq_restore(flags) (flags) > > +#define local_nest_irq_disable() > > +#define local_nest_irq_enable() > > + > > This list is going to increase with RT support in SLUB? Argh. > AFAIK, there is no difference between local irq save/restore in mainline VS -RT. The same applies to preempt disable/enable. The only thing we have to make sure is that the irq disable and preempt disable code paths are short and O(1). > > > Index: linux-2.6-lttng/include/asm-i386/local.h > > =================================> > --- linux-2.6-lttng.orig/include/asm-i386/local.h 2007-09-04 15:28:52.000000000 -0400 > > +++ linux-2.6-lttng/include/asm-i386/local.h 2007-09-05 08:49:19.000000000 -0400 > > @@ -194,6 +194,23 @@ static __inline__ long local_sub_return( > > }) > > #define local_inc_not_zero(l) local_add_unless((l), 1, 0) > > > > +#define local_enter_save(flags) \ > > + do { \ > > + (flags); \ > > + preempt_disable(); \ > > + } while (0) > > > > +#define local_exit_restore(flags) \ > > + do { \ > > + (flags); \ > > + preempt_enable(); \ > > + } while (0) > > > This does not result in warnings because a variable is not used or used > uninitialized? Because the variable is not used at all if I don't put the "(flags)" (gcc warns about this). I'm glad that some of the proposed changes may help. I'll let the cmpxchg_local patches sleep for a while so I can concentrate my efforts on text edit lock, immediate values and markers. I think what we'll really need for the cmpxchg_local is two flavors: one that is as atomic as possible (for things such as tracing), and the other one the fastest possible (potentially using irq disable). A lot of per architecture testing/fine tuning will be required though, and I don't have the hardware to do this. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68